The Element Set and Element names are output for display on the site in application/views/scripts/items/item-metadata.php

By default, those names come directly from the database. You could change them in the database, but that's a potentially bad idea, since you'd have to be sure to pick up any references to "Dublin Core" or the element names elsewhere in the code.

A better idea is probably to add some code to item-metadata.php that allows you to add an array of English-to-Dutch mappings for the Element and Set names. So, where the code currently directly outputs $elementName, you could replace that with a lookup to your array, like $dutchElementNames[$elementName].

All that being said, I'll note that the next Omeka milestone, version 1.5, is slated to contain mechanisms to allow people to much more easily translate Omeka into other languages, so you wouldn't need to delve in to the code like this.